1. Field of the Invention
The present invention relates to an internal combustion engine provided with a decompression mechanism incorporated into a camshaft included in a valve train and disposed in a valve chamber. The internal combustion is intended for use as, for example, an outboard engine.
2. Description of the Related Art
An internal combustion engine intended for use as an outboard engine disclosed in, for example, JP2000-227064A (FIGS. 4 and 5) is a two-cylinder internal combustion engine provided with a decompression mechanism. This two-cylinder internal combustion engine is provided with a camshaft disposed in a cam chamber defined by a cylinder head and a cylinder head cover, cams formed on the camshaft to operate intake valves and exhaust valves, rocker arms driven for a rocking motion by the cams, a decompression lever mounted on the camshaft so as to be turnable in a vertical plane under the cams for operating the exhaust valves, and a fuel pump. The internal combustion engine disclosed in JP2000-227064A is provided with flanges at the upper and the lower ends of the camshaft to restrain the camshaft from axial movement.
A three-cylinder internal engine, intended for use as an outboard engine, disclosed in JP3-3904A is provided with a camshaft supported in a plurality of bearings on a cylinder head, cams formed on the cam shaft to rock rocker arms (hereinafter referred to as xe2x80x9cvalve camsxe2x80x9d), and a fuel pump that is driven by a pump driving mechanism including an eccentric cam formed on the camshaft at a position between the lowermost valve cam and the lowermost bearing, and a rod operated by the eccentric cam. The fuel pump is attached to a side surface of the cylinder head. The rod has a first end in contact with the eccentric cam and a second end in contact with a contact part of an actuating member included in the fuel pump. The first and the second ends of the rod are at substantially the same positions with respect to a direction parallel to the axis of the camshaft, and the eccentric cam and the contact part coincide with each other with respect to the direction parallel to the axis of the camshaft. The eccentric cam is fitted in a groove formed in a thrust holder formed integrally with a bearing cap holding the lowermost bearing with the opposite side surface thereof in contact with the opposite side surfaces of the groove of the thrust holder. Thus, the eccentric cam is restrained from axial movement by the groove of the thrust holder. When the fuel pump is driven by the eccentric cam formed on the camshaft (hereinafter, referred to as pump cam), the first and the second ends of the rod, i.e., a cam follower for transmitting the driving force of the pump cam to the fuel pump, are at the same positions with respect to the direction parallel to the axis of the camshaft as mentioned in JP3-3904A. When it is desired to incorporate the eccentric cam, the rod and the fuel pump mentioned in JP3-3904A into the prior art internal combustion engine disclosed in JP2000-227064A, the fuel pump protrudes down greatly from the cylinder head, the length of the camshaft needs to be increased to avoid interference between the rod, and members and parts in the valve chamber, such as bosses through which head bolts are extended to fasten the cylinder head to the cylinder block. Consequently, the length of the cam chamber must be increased.
When the valve cam for the intake valve, the valve cam for the exhaust valve and the decompression lever for each cylinder, and the pair of flanges, and the pump cam for driving the fuel pump are formed on the camshaft according to the technique disclosed in JP2000-227064A, the camshaft inevitably become long to form the pump cam and the flanges in different parts of the camshaft and, consequently, the cam chamber containing the camshaft inevitably becomes long.
The internal combustion engine disclosed in JP3-3904A is not provided with any decompression mechanism, the thrust holder is disposed between the lowermost bearing and the second lowermost bearing and between the valve cams for the lowermost cylinder and the eccentric cam. Therefore, the length of the camshaft must be increased to incorporate a decompression mechanism into the lowermost cylinder.
The present invention has been made in view of the foregoing circumstances and it is therefore an object of the present invention to suppress the increase of the length of a camshaft disposed in a valve chamber and provided with a pump cam and a decompression mechanism, the axial protrusion of the fuel pump from a valve chamber forming member, and the increase of the axial dimension of a valve chamber, and to provide a compact internal combustion engine.
According to the present invention, an internal combustion engine comprises: a camshaft interlocked with a crankshaft; a valve chamber forming member forming a valve chamber for containing the camshaft; a valve train arranged in the valve chamber to open and close intake and exhaust valves; decompression mechanisms arranged in the valve chamber to open the intake or the exhaust valves during a compression stroke; a fuel pump having an actuating member extending in the valve chamber, and attached to the valve chamber forming member; a plurality of bearings arranged in the valve chamber to support the camshaft; journals formed in the camshaft and supported by the bearings, the number of the journals being equal to that of the bearings; wherein a pump cam for driving the actuating member through a cam follower is formed adjacently to the end journal at one axial end of the camshaft among the journals on the camshaft, a specific one of the decompression mechanisms and the end journal are disposed on the opposite sides, respectively, of the pump cam with respect to an axial direction, the camshaft is provided with a valve cam for opening and closing the intake or the exhaust valve to be opened by the specific decompression mechanism, the specific decompression mechanism is disposed between the pump cam and the valve cam, the cam follower has a contact part in contact with the pump cam and an acting part in contact with the actuating member at a position nearer to the valve cam than the contact part with respect to the axial direction.
Since the acting part of the cam follower that transmits the driving force of the pump cam to the actuating member of the fuel pump is farther from the end journal than the contact part, the actuating member, hence the fuel pump, can be disposed apart from the end journal, hence from an end wall of the valve chamber forming member, with respect to the axial direction. Moreover interference between the actuating member and members at the same position as the pump cam with respect to the axial direction can be avoided.
Consequently, the present invention has the following effects. Since the specific decompression mechanism is adjacent to the end journal at the axial end among the plurality of journals and is disposed between the pump cam for driving the actuating member through the cam follower, and the valve cam for opening and closing the intake or the exhaust valve opened by the decompression mechanism, and the cam follower has the contact part in contact with the pump cam and the acting part in contact with the actuating member at a position nearer to the valve cam than the contact part with respect to the axial direction, the actuating member, hence the fuel pump, can be spaced from the end wall of the valve chamber forming member with respect to the axial direction. Since interference between the actuating member and members at the same position as the pump cam with respect to the axial direction can be avoided, the increase of the length of the camshaft and the axial protrusion of the fuel pump from the valve chamber forming member can be suppressed, and thereby the internal combustion engine is compact.
In the internal combustion engine according to the present invention, the pump cam may be adjacent to a specific one of the plurality of bearings, and the specific decompression mechanism may be disposed opposite the specific bearing relative to the pump cam and adjacently to the pump cam to form a thrust bearing member for restraining the camshaft from axial movement.
Since the pump cam serves as a thrust-bearing member, the camshaft is shorter than a camshaft provided with a pump cam and a separate thrust-bearing member, and the decompression mechanism can be disposed adjacently and close to the pump cam.
Such construction provides the following effects. The pump cam for driving the fuel pump serves as the thrust bearing member adjacent to the specific bearing among the bearings supporting the plurality of journals of the camshaft and capable of restraining the axial movement of the camshaft, an axial space is available because the decompression mechanism is disposed opposite the specific bearing and adjacently to the pump cam, the increase of the length the camshaft provided with the pump cam and the decompression mechanisms can be suppressed because the decompression mechanism can be disposed near the pump cam, and thereby the enlargement of the valve chamber can be suppressed and the internal combustion engine can be formed in a short axial length.
In the internal combustion engine according to the present invention, the pump cam may be disposed so as to be in contact with the specific one of the plurality of bearings to make the pump cam serve as the thrust bearing member for restraining the camshaft from axial movement, the pump cam and the valve cams associated with a cylinder included in the internal combustion engine, and the specific decompression mechanism may be disposed between the specific bearing and the bearing axially adjacent to the specific bearing, and the valve cams or the specific decompression mechanism may be disposed axially opposite the specific bearing with respect to the pump cam so as to be adjacent to the pump cam.
Since the pump cam thus serves also as a thrust-bearing member, an axial space along the camshaft is formed between the specific bearing and the bearing adjacent to the specific bearing disposed on the opposite sides, respectively of the cylinder. Since the valve cams or the decompression mechanism is disposed axially adjacently to the pump cam between the specific bearing and the bearing adjacent to the specific bearing, the valve cams or the decompression mechanism can be disposed axially close to the pump cam.
Such construction provides the following effects. The pump cam for driving the fuel pump serves as the thrust bearing member disposed adjacently to the specific one of the bearings supporting the plurality of journals of the camshaft to restrain the camshaft from axial movement, the axial space is formed along the camshaft between the specific bearing and the bearing adjacent to the specific bearing disposed on the opposite sides, respectively, of the cylinder by disposing the pump cam and the valve cams associated with the cylinder, and the decompression mechanism between the specific bearing and the bearing adjacent to the specific bearing and disposing the valve cams or the decompression mechanism opposite the specific bearing and adjacently to the pump cam, and the valve cams and the decompression mechanism can be disposed near the pump cam. Thus, the increase of the length the camshaft provided with the pump cam and the decompression mechanisms can be suppressed, and thereby the enlargement of the valve chamber can be suppressed and the internal combustion engine can compactly be formed.
In this specification, unless otherwise specified, xe2x80x9caxial directionsxe2x80x9d signifies a direction parallel to the axis of the camshaft.